Astronomy in Alexandria

Astronomy in Alexandria

In the second century A.D. the Greeks combined their celestial theories with observations translated to planes.

Astronomers Nicea Hipparchus and Claudio Ptolemy They determined the positions of some 1,000 bright stars and used this star map as the basis for measuring planetary movements.

When replacing the spheres of Eudoxo by a more flexible system of circles, they raised a series of eccentric circles, with the Earth near a common center, to represent the general movements towards the East around the zodiac at different speeds of the Sun, the Moon and the planets.

To explain the periodic variations in the speed of the Sun and the Moon and the setbacks of the planets, they said that each of these bodies revolved uniformly around a second circle, called the epicycle, whose center was located in the first. By the appropriate choice of the diameters and the speeds of the two circular movements attributed to each body, its observed movement could be represented. In some cases a third body was needed to square the calculations.

Ptolemy compiled the astronomical knowledge of his time in the thirteen volumes of the "Almagesto". He exhibited a system where the Earth, in the center, was surrounded by glass spheres from the other 6 known stars. This scheme is known as the Geocentric system from Ptolemy.

The earth did not exactly occupy the center of the spheres and the planets had an epicycle (system created by Apollonius of Pergamum and perfected by Hipparchus) whose axis was the line of the orbit that revolved around the earth called deferential.

As the planet revolves around its epicycle, it approaches and moves away from the earth, sometimes showing retrograde motion. This system allowed to make predictions of the planetary movements, although it had a very poor precision. Despite this, it was popularized and accepted rather than as a true model as a useful mathematical fiction. It is estimated that the Ptolemaic universe only average 80 million kilometers.

Another thinker who, like Ptolemy, kept alive the tradition of Greek astronomy in Alexandria in the first centuries of the Christian era, was Hipatia, a disciple of Plato. He wrote comments on mathematical and astronomical topics and is considered the first scientist and philosopher of the West. The movie "Agora" recreates its history.

Other achievements of Astronomy in Alexandria were the calculation of the circumference of the earth by Eratosthenes and the first measurements of the distances to the Sun and the Moon. Stellar catalogs such as those of Hipparchus of Nicea and the discovery of the precession of the equinoxes were designed.

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